Yea, the Vf spec is of just passing interest, we know that a 5 volt signal will fully turn on the positive clamping protection diode on a input digital pin on a 3.3 volt powered board, what we want to know is how much continous current the diode can withstand, that is the datasheet spec of interest.

It seems as if Atmel is a bit candid about this limitation. There is an AVR application note on zero crossing however, where they use a digital pin directly connected to mains. In this note, maximum continues forward current is quoted as 1mA. The relevant text reads as follows:

Quote

The series input resistor is a 1 M? resistor. It is not recommended that the clampingdiodes are conducting more than maximum 1 mA and 1 M? will then allow a maximumvoltage of approximately 1,000V.

Any voltage higher than 1,000V would probably be spikes or surges. The clampingdiodes are able to handle spikes for a short period of time but not surges. The applicationnote will not go into how to protect against surges, but simply recommendimplementing protection against surges in the design.

Yea, the Vf spec is of just passing interest, we know that a 5 volt signal will fully turn on the positive clamping protection diode on a input digital pin on a 3.3 volt powered board, what we want to know is how much continous current the diode can withstand, that is the datasheet spec of interest.

It seems as if Atmel is a bit candid about this limitation. There is an AVR application note on zero crossing however, where they use a digital pin directly connected to mains. In this note, maximum continues forward current is quoted as 1mA. The relevant text reads as follows:

Quote

The series input resistor is a 1 M? resistor. It is not recommended that the clampingdiodes are conducting more than maximum 1 mA and 1 M? will then allow a maximumvoltage of approximately 1,000V.

Yes, that 1ma maximum recommended continuous current rings a bell. So for a 5 volt signal wired to a 3.3 volt input pin, 1700 ohms or more series current limiting resistor should be OK.Lefty

Any voltage higher than 1,000V would probably be spikes or surges. The clampingdiodes are able to handle spikes for a short period of time but not surges. The applicationnote will not go into how to protect against surges, but simply recommendimplementing protection against surges in the design.